I. Field of the Invention
This invention relates generally to electric motor control apparatus, and more specifically to an adaptive, closed-loop control system for improving the efficiency of AC motors subjected to varying mechanical load conditions.
II. Discussion of the Prior Art
In the U.S. Pat. No. 4,052,648 to Nola, there is described a non-adaptive control arrangement for an AC induction motor in which applied voltage and motor current are sampled, with the phases of the samples being compared such that a control signal, representative of the phase difference, is obtained. This control signal is then employed to vary the duty cycle of the applied voltage to thereby regulate phase difference and improve the power factor to a more optimum state when previously the motor had been operating at a less than optimum relationship between line voltage and motor load. In the Nola arrangement, it is required that the optimum phase lag for the particular motor in question to be known and this optimum value varies from motor to motor. The optimum phase lag value is manually entered by adjusting a potentiometer so that the error signal will be zero when the motor is operating at its optimum phase lag condition. When the mechanical load on the motor changes, the phase angle between the voltage and current waves will also change. This action results in a control signal being developed for returning the phase angle to the present optimum value.
The system of the present invention, while related to the Nola arrangement, differs therefrom in that it is a fully adaptive, closed-loop controller for varying the duty cycle of the voltage applied to the motor being controlled. By increasing the time delay from the zero-crossing of the applied voltage signal to the point where that voltage is applied to the motor, it is possible to improve motor efficiency. Then, if the motor's mechanical load undergoes a change, the firing angle also changes, the system serving to adaptively track the optimum operating condition. Further, the system of the present invention offers the advantage of providing the optimal reduced voltage to the motor even as the motor's mechanical load changes. The apparatus of the Nola Patent being non-adaptive, it is unable to track a varying optimum value, but instead, merely functions to return an operating parameter to a preset operating point. Stated otherwise, instead of zeroing in on a preset value of voltage, current or phase angle, the controller of the present invention seeks out the optimum operating point upon changes in the mechanical load on the motor's shaft. In this regard, the system of the present invention bears a relationship to the control scheme described in the Schweitzer U.S. Pat. No. 3,142,967 as well as the later Schweitzer et al U.S. Pat. Nos. 4,026,251 and 4,130,863 in that in each of these patents, an optimum setting of a control parameter is sought out by first introducing a relatively slow acting perturbation termed a "dither" of the control parameter setting and then noting wheher the change thus introduced results in improved output performance of the mechine being controlled. If the dither in a given direction of the machine parameter does, in fact, result in improved performance, the controller acts to further adjust that control parameter in the same direction. However, if a degradation in performance is noted, then the machine control parameter is adjusted in the opposite direction until improved performance is again achieved.